description = [[
Determines whether the encryption option is supported on a remote telnet server.  Some systems (including FreeBSD and the krb5 telnetd available in many Linux distributions) implement this option incorrectly, leading to a remote root vulnerability. This script currently only tests whether encryption is supported, not for that particular vulnerability. 

References:
* FreeBSD Advisory: http://lists.freebsd.org/pipermail/freebsd-announce/2011-December/001398.html
* FreeBSD Exploit: http://www.exploit-db.com/exploits/18280/
* RedHat Enterprise Linux Advisory: https://rhn.redhat.com/errata/RHSA-2011-1854.html
]]

---
-- @usage
-- nmap -p 23 <ip> --script telnet-encryption
--
-- @output
-- PORT   STATE SERVICE REASON
-- 23/tcp open  telnet  syn-ack
-- | telnet-encryption: 
-- |_  Telnet server supports encryption
--
--

categories = {"safe", "discovery"}

require 'shortport'

portrule = shortport.port_or_service(23, 'telnet')

author = "Patrik Karlsson, David Fifield, Fyodor"
license = "Same as Nmap--See http://nmap.org/book/man-legal.html"

local COMMAND = {
	SubCommand = 0xFA,
	Will = 0xFB,
	Do = 0xFD,
	Dont = 0xFE,
	Wont = 0xFC,	
}

local function processOptions(data)
	local pos = 1
	local result = {}
	while ( pos < #data ) do
		local iac, cmd, option
		pos, iac, cmd = bin.unpack("CC", data, pos)
		if ( 0xFF ~= iac ) then
			break
		end
		if ( COMMAND.SubCommand == cmd ) then
			repeat
				pos, iac = bin.unpack("C", data, pos)
			until( pos == #data or 0xFF == iac )
			pos, cmd = bin.unpack("C", data, pos)
			if ( not(cmd) == 0xF0 ) then
				return false, "Failed to parse options"
			end
		else
			pos, option = bin.unpack("H", data, pos)
			result[option] = result[option] or {}
			table.insert(result[option], cmd)
		end
	end
	return true, { done=( not(#data == pos - 1) ), cmds = result }
end

action = function(host, port)

	local socket = nmap.new_socket()
	local status = socket:connect(host, port)
	local data = bin.pack("H", "FFFD26FFFB26")
	local result

	socket:set_timeout(7500)
	status, result = socket:send(data)
	if ( not(status) ) then
		return ("\n  ERROR: Failed to send packet: %s"):format(result)
	end

	repeat
		status, data = socket:receive()
		if ( not(status) ) then
			return ("\n  ERROR: Receiving packet: %s"):format(data)
		end
		status, result = processOptions(data)
		if ( not(status) ) then
			return "\n  ERROR: Failed to process telnet options"
		end
	until( result.done or result.cmds['26'] )

	for _, cmd in ipairs(result.cmds['26'] or {}) do
		if ( COMMAND.Will == cmd or COMMAND.Do == cmd ) then
			return "\n  Telnet server supports encryption"
		end
	end
	return "\n  Telnet server does not support encryption"
end
